Only impose minimal height for buttons with non-empty label in wxMSW.

Only impose minimal height for buttons with non-empty label in wxMSW.

wxBU_EXACTFIT in wxMSW only affected the width of the button but not its
height, which was still made as big as the standard button size as otherwise
the button text was rendered badly. However there is no reason to do this for
the buttons without any text, so let wxBU_EXACTFIT be really exact, in both
directions, in this case.

Also document this rather non obvious wxBU_EXACTFIT behaviour.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73461 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775